courtesy of Jens Lyck
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Technical

Carburetor - drawings, charts, and a how-to guide.

It's not that hard - and yet! Most people understands the basic function of the carburetor, but it's a long hard road to learn to adjust it properly.

If your bike is in fairly standard trim start from the factory settings as found below. From here you can experiment with jets and needles. Its quite cheap on the Dell Orto's.

Note that all the basic stuff like ignition timing, valve adjustment and synchronization of the two carbs must be properly done before you start playing with the jets. Otherwise its just a waste of time.

If you don't feel like using a lot of time to learn this, I suggest you pay a qualified mechanic to do it (look carefully), and then stick to emptying the float chambers and do the synchronization once in a while.

Dell Orto's are very easy to work on. The necessary information can be found on these pages, but remember: YOU'RE RESPONSIBLE FOR WHAT YOU'RE DOING.

Table of contents

 

Factory settings

See how it was jetted from the factory. Usually, older bikes have had a number of weird owners with weird ideas about how to jet a bike. So start by checking the jets and needles.

MC Carburetor Idle jet Main jet needle Clip position (From top) Needle jet Slide
V7 Sport, 750S, S3, 850T VHB 30 50 142 V-9 2 265 40
T3, T4 VHB 30 50 120 V-9 2 265 40
Le Mans 1 PHF 36 60 135 K-5 2 265 60/1
Le Mans 2 PHF 36 60 140 K-5 2 265 60/1
Le Mans 3 PHF 36 50 115 K-18 3 268 60/3
Le Mans 4 PHM 40 57 145 K-19 3 268 60/5
Le Mans 5, Big valve California 3 PHM 40 57 145 K-19 3 268 60/5
California 2, SP2 VHB 30 50 125 V-9 2 265 40
T5, CX100, 1000SP, Convert, G5 VHB 30 50 130 V-9 2 265 40
California 2, SP2, T5, CX100, 1000SP, Convert, G5 PHF 30 50 125 K-23 3 264 50/3
California 3, Mille GT PHF 30 50 130 K-23 3 264 50/3
1000S PHM 40 57 145 K-19 3 266 60/5
Strada 1000, 1000S, SP3, California 3 PHF 36 50 130 K-18 3 268 60/3
California 1100 PHF 36 50 130 K-18 2 266 60/3
Sport 1100 PHM 40 57 152 K-18 3 266 60/5

Note that LM3 uses a special version of the PHF36 with a different atomizer that allows for the use of smaller main jets.

"Explosion" drawing

Including spare parts numbers

Diag No. Part No. Description Sizes/options available
1 8639 Slide 401,501,503,505,601,603 & 605
2 8530 Needle K1 to K95
3 8540 Atomizer AB 258,60,62,64,65,66,68,70,72,75,78,80,90 & 300
3 12599 Atomizer DR 266,268,270 & 272.
4 9980 Idle emulsion tube Not fitted to all models
5 6413 Main Jet 56 to 330, in steps of 1,2 & 3. Please ask.
6 1486 Idle jet (short) 30 to 198, in steps of 1,2 & 3. Please ask
7 7746 Choke jet 45,50,55,60,65,70,75,80,85 & 90
8 7851 Pump jet 30,33,35,38,40,42,45,48,50,55,60,65,70,75,90,150
9 10375 Float needle valve 150, 170, 200, 225
9 8649 Float needle valve 250, 270, 300, 350, 400
10 7450 Float .1, .2 & .3
11 6429 Top cover screw  
12 1581 Spring washer  
13 8826 Top paper gasket  
14 8542 Screw  
15 11779 Slide rod  
16 11798 Top casting  
17 8726 Spindle seal  
18 8727 Spindle seal cover  
19 8535 Pump arm pin Metal top
19 10924 Pump arm pin Plastic top
20 9383 Spring  
21 11777 Slide rod  
22 10451 circlip  
23 8941 Throttle lever  
24 11773 Needle washer  
25 9596 Needle circlip  
26 8633 Top cover O ring  
27 3718 Washer  
28 11776 Slide rod  
29 11774 Spring guide  
30 11786 External return spring  
31 11775 Spring guide  
32 11787 Throttle cable support  
33 6435 Screw  
34 4957 Spring washer  
35 11788 Left hand throttle support  
36 8825 Dome top cover  
37 9042 Allen screw  
38 5011 Spring washer  
39 8830 Cable arm  
40 8725 Spindle seal  
41 10069 Slide rod guide  
42 7415 Pump arm spring  
43 8426 Pump arm  
44 3698 90 Degree cable elbow  
44 3600 70 Degree cable elbow  
44 9330 40 Degree cable elbow  
45 1692 Adjuster locknut (for 1481)  
46 1481 Cable adjuster  
47 1476 Rubber cable cap  
48 1104 Cable adjuster  
49 1691 Adjuster locknut (for 1104)  
50 3128 Choke support  
51 3133 Choke return spring  
52 3238 Choke piston  
53 11650 Long choke body screw  
54 8711 Choke body screw  
55 8887 Choke housing  
56 8888 Choke housing gasket  
57 11651 Choke housing spacer (LM)  
57 7749 Mixture screw "standard"  
58 11084 Mixture screw "Guzzi"  
59 9336 Mixture screw spring  
60 8260 Mixture screw washer  
61 8678 Mixture screw O ring  
62 10747 Pump jet holder  
63 6426 Pump jet holder fibre washer  
64 6173 Pump jet O ring  
65 7673 Throttle stop screw  
66 4670 Throttle stop screw spring  
67 4650 Washer  
68 7540 O ring  
69 10800 Pump non return valve  
70 8556 Pump cover  
71 8598 Adjuster screw O ring  
72 8599 Pump adjuster locknut  
73 8539 Pump adjustment screw  
74 8428 Screw  
75 7626 Diaphagm spring  
76 8555 Pump diaphragm  
77 9278 Diaphragm gasket  
78 8885 Main jet holder  
79 11785 Operating lever, RH  
80 11799 Top casting  
81 6109 Fuel banjo filter  
82 9250 Plastic fuel union  
82 6273 Metal fuel union  
82 7890 Double fuel union  
83 4568 Fuel union bolt  
84 8557 Non return valve  
85 6288 Needle valve fibre washer  
86 7451 Float bowl O ring  
87 7346 Float pivot pin  
88 10557 Float bowl  
89 4057 Float bowl nut seal  
90 10238 Float bowl nut  
91 52560 PHM N, cranked top  
91 52520 PHM A,B,N,V gasket set  
91 52544 PHM..H,L,M,P,R,T,S,Z gasket set  
91 52545 PHM A,B,N,V gasket set  
92 4052 Fuel union seal  
93 9573 Short plastic trumpet  
94 8673 Long trumpet with wire gauze  
95 8128 Clamp nut  
96 8127 Clamp bolt  
97 8528 Clamp  
98 8596 PHM38 R90S mounting sleeve  
99 10923 Plastic top  
100 8550 Slide spring 0.7kg/70mm  
100 8532 Slide spring 1.1kg/70mm  
100 9389 Slide spring 1.4kg/70mm  
101 8525 Needle clip  

Numbers on jets and needles

Do the tiny numbers have a purpose ?

Jets:

Its quite simple - the number on the jet tells you the size of the hole in the jet, measured in 1/100 mm's. So a 60 jet has a bore of 0,6mm and a 268 needle jet is 2,68mm inside. Note that the thread and the external size of the jet depends of its location in the carburetor. So when you order a jet, you'll have to state if you want a main jet, idle jet, etc, AND the size of the jet.

Needle:

This is a bit more tricky. There's so many significant dimensions on a carburetor needle that it wouldn't make sense to identify the needle with a number referring to a single dimension on the needle. So they have apparently random numbers, and to see the differences between two needles its necessary to use the chart below.

Type Ø-A Ø-B C Ø-D E F G
K-1 2,45 1,75 37 - - - -
K-2 2,45 1,75 42 - - - -
K-3 2,50 1,5 39 - - - -
K-4 2,45 1,5 39 - - - -
K-5 2,45 1,5 37 - - - -
K-6 2,45 1,75 39 - - - -
K-7 2,45 1,25 39 - - - -
K-8 2,50 1,5 37 - - - -
K-9 2,45 1,5 42 - - - -
K-11 2,50 1,25 39 - - - -
K-12 2,48 1,75 32 - - - -
K-13 2,45 1,25 38 - - - -
K-14 2,48 1,75 33 - - - -
K-15 2,50 0,6 26 - - - -
K-16 2,50 1,75 39 - - - -
K-17 2,42 1,75 40 - - - -
K-18 2,50 1,4 38 - - - -
K-19 2,50 1,4 40 - - - -
K-20 2,50 1,4 42 - - - -
K-21 2,50 1,8 38 - - - -
K-22 2,50 1,8 40 - - - -
K-23 2,50 1,8 42 - - - -
K-24 2,50 1,2 38 2,13 18 - -
K-25 2,50 1 36 2,15 18 - -
K-27 2,50 1,8 44 - - - -
K-28 2,50 1,8 41 - - - -
K-29 2,45 1,25 42 - - - -
K-30 2,50 1,4 36 2,15 18 - -
K-32 2,48 1,7 44 - - - -
K-33 2,50 1,8 44 - - - -
K-34 2,50 1,4 40 2,11 18 - -
K-35 2,50 1,4 43 - - - -
K-36 2,50 1,4 38 2,17 20 - -
K-37 2,50 1,4 39 2,12 18 - -
K-38 2,50 1,4 38 2,13 18 - -
K-39 2,48 1,45 36 2,28 26 - -
K-40 2,50 1,4 40 2,18 22 - -
K-41 2,50 1,4 40 2,14 22 - -
K-42 2,50 1,4 38 2,16 22 - -
K-43 2,50 1,4 42 2,16 26 - -
K-44 2,50 1,4 39 2,06 20 - -
K-45 2,48 1,3 36 2,28 26 - -
K-46 2,50 1,4 40 2,15 20 - -
K-48 2,48 1,6 36 2,25 25 11 1,60
K-49 2,50 1,4 39 2,20 26 - -
K-50 2,50 1,4 39 2,27 26 - -
K-52 2,50 1,6 36 2,25 25 11 1,60
K-53 2,52 1,6 36 2,25 25 11 1,60
K-54 2,48 1,5 40 2,108 18 - -

Jetting guide

Written for Japanese carbs, but principles are all the same.

Quick Jetting Guide for Mikuni and Keihin CV (carburetors with movable jet needles)

The following tips and insights are for your information only. These will not make you an expert tuner. We recommend that if you are a novice, you consult a professional mechanic or technician for your specific needs. The author cannot be responsible for your interpretation and usage of this information.

  1. IDLE: Set idle speed to proper r.p.m. by adjusting the IDLE SPEED SCREW. Turn the AIR SCREW to achieve the highest speed and best response. After adjustment has been made reset the IDLE SPEED SCREW to the proper r.p.m.
  2. OFF IDLE to 1/4 THROTTLE: The SLOW JET and AIR SCREW are most effective in this range When you want a richer mixture use a larger SLOW JET or turn the AIR SCREW in. The opposite holds true for a leaner mixture
  3. 1/4 to 3/4 THROTTLE: The JET NEEDLE is the most effective component in this range. Raising the needle by lowering the chip position at the top of the needle will richen the mixture. Lowering the needle will lean the mixture.
  4. WIDE OPEN THROTTLE: Changing the MAIN JET affects this range. Select the size which offers the best wide open throttle. performance, then install one size larger MAIN JET for ideal engine durability.

JETTING RANGE EFFECTIVENESS CHART

THROTTLE OPENING

Jetting Your Carb Circuits

To visualize how the various circuits overlap, please refer to the jetting chart. Always remember to change one carburetor component at a time and keep a record of your changes.

ZERO THROTTLE OPERATION (IDLE CIRCUIT)

IDLE SCREW:

Depending on your type of riding, adjust the minimum idle speed to desired RPM making sure the engine is up to operating temperature. If you do not desire any idle, make sure you turn in the adjusting screw just enough so the engine will not idle. This is especially important on Keihin PJ Series carbs in that the idle adjust knob (#4 in illustration) cannot be completely closed. Such an adjustment will result in a sluggish response off idle.

AIR/FUEL ADJUSTMENT SCREW:

The carburetor pictured in the exploded view uses an air adjustment screw (#5 in illustration) that is located "upstream" of the throttle valve (slide) and meters air. Turning the air screw counter-clockwise leans the mixture off idle. Some carburetors have this screw located "downstream" of the throttle valve, in which case, the screw meters fuel and opening the screw (counter-clockwise) results in a richer mixture. The idle screw usually has a range of one to two turns out from fully closed. If you need to adjust above or below this range, then the fuel jet will probably need to be replaced with a richer/leaner jet as required. Consult your owner's manual for the standard setting.

1/8 TO 1/4 THROTTLE

SLOW JET AND THROTTLE VALVE CUTAWAY:

Note - keep in mind that the idle adjust screw (air/fuel screw) gives a good indication of a properly sized slow jet (#6 in illustration). The slow jet calibrates the mixture from both the idle bypass and the idle orifice in the jet block. If the idle screw is properly adjusted, but the engine does not have good response when the throttle is wicked open, it is usually a sign of a lean mixture and the slow jet will need to be replaced with one size larger (richer) and the air/fuel screw re-adjusted. Consequently, if the throttle is only partially opened, such as in a trailing throttle situation, and the bike tends to load up, emitting a deep tone when the throttle is returned to full open, it is usually a sign of a rich slow jet. If the slow jet does not clean up this part of the circuit, the slide can be substituted for one with a different cutaway. The higher the number, the larger the cutaway will be, allowing more air to the jet block/nozzle screen leaning the mixture and, conversely, a smaller cutaway will richen the mixture with a greater effect up to 1/4 throttle.

1/4 TO 3/4 THROTTLE

JET NEEDLE:

The jet needle (#2 in illustration) is comprised of five major elements.

  1. Straight diameter section - In Keihin carburetors, either the last two digits or last letter denotes the diameter of the needle. The higher the last two digits, the leaner the needle and the lower the letter, the richer the needle. By going to a thinner needle, there is a larger area between the jet needle and the needle jet supplying a richer mixture.
  2. Length of the straight section - This determines at which point the needle taper will start relative to the clip position. If you have to run a needle in the highest clip position, a needle with a longer straight section should be used.
  3. Needle Clip Position - This works in conjunction with the length of the straight section. If the engine is too rich above 1/4 throttle, raising the needle clip (#1 in illustration) will lean the mixture.
  4. Needle Taper - A larger taper will result in a leaner mixture in the first half of the taper and a richer mixture in the last half of the needle. For example, a 1.34 taper will be richer in the first half and leaner in the second half of the taper than a 1.45 taper needle.
  5. Number of tapers - The needle can have one or more tapers; the number of tapers is not usually changed.

NEEDLE JET:

The needle jet/nozzle controls the fuel/air mixture up to 3/4 throttle. How it overlaps with the jet needle depends on the jet orifice inner diameter, air bleed holes and type of nozzle screen. Most modern Japanese carburetors use a fixed needle jet/nozzle assembly which cannot be removed. It your carburetor has a removable needle jet/nozzle, please contact the manufacturer in order to decipher the nozzle code. It is advisable not to calculate how rich/lean the needle jet is by using exclusively the nozzle inside diameter to needle outside diameter discharge area.

WIDE OPEN THROTTLE

MAIN JET:

The best track side method to determine the size of the main jet (#7 in illustration) is to fully load the engine on a long straightaway or hill. At the end of the stretch, chop the throttle and hit the kill button simultaneously. Now pull the spark plug. The parts of the plug you should be looking at are the positive electrode and last 1/4 of the ceramic insulator. Best power will usually result in a very light tan colored insulator tip and dark colored ring around the tip of the electrode. The electrode itself should have fairly sharp edges. For example, if the ceramic insulator has a nice tan coloring but the electrode has a white ring around the tip and the plug is of the correct heat range, then you can easily run a size larger on the main jet.

When jetting your main jet, try to remember to jet for the best power in a specific situation. As you gain experience and knowledge, you will be able to use other methods to determine your jetting. A good tuner can "feel" most of the circuits by slowly reving a parked bike, or just by looking at the color of the unpainted pipe and silencer.